I think the "My prescriptions read like ransom notes" bit means that he is in such serious depression, or nervous state seeing as it's mentioned in the above line "Why are you so nervous, boy", that he needs tons of meds, or expensive ones at least, so it appears like a ransom note due to the cost.
He's comparing it to love letters because, like doftpunk101 said, because they're fairly long. He may also be comparing them because in a love note someone may write all the reasons why they love a person. So in his prescriptions, it's kind of like all the different meds he has to use because of the girl. :)
I think the "My prescriptions read like ransom notes" bit means that he is in such serious depression, or nervous state seeing as it's mentioned in the above line "Why are you so nervous, boy", that he needs tons of meds, or expensive ones at least, so it appears like a ransom note due to the cost. He's comparing it to love letters because, like doftpunk101 said, because they're fairly long. He may also be comparing them because in a love note someone may write all the reasons why they love a person. So in his prescriptions, it's kind of like all the different meds he has to use because of the girl. :)